Magnetic recording is the preferred method of information storage when data must be capable of being read immediately after writing or where the data is to be processed by a machine.
Since 1950, rapid improvements in the field of magnetic recording have been achieved and, as the continuing advances in information handling technology have led to the requirement for data storage and retrieval systems capable of handling extremely large volumes of information, one of,the main areas of the research into magnetic recording systems has been to increase the information density or storage capacity of the media.
Magnetic data storage or recording apparatus typically use magnetic recording media in which data is stored in "tracks". In rigid type magnetic disk drives, track densities of between 20 and 40 tracks per mm (500 and 1000 tracks per inch) are typically used. The track density of magnetic storage disks for conventional floppy disk drives is typically from 2 to 5 tracks per mm (48 to 135 tracks per inch).
Most of the currently available systems of magnetic recording employ particulate magnetic recording media in which small, discrete magnetic particles (referred to as the "magnetic pigment or paint") are dispersed in an organic binder matrix. The degree of storage that is possible is related to the packing density of the magnetic particles in the media, and the amount of information that can be recorded on such materials is fast approaching its theoretical limit.
The problem with magnetic disks carrying a large amount of information in densely packed magnetic tracks is that it is difficult to ensure proper positioning of the read/write (or transducer) head to a desired track and to ensure accurate tracking once the selected track has been found. As track density increases, a narrower read/write head is required for reading/writing the data in the tracks, and evermore precise tracking to allow the correct track to be located with the read/write head positioned accordingly. Moreover, it is important not only that the tracking technique be reliable but it is also highly desirable that it be easy and inexpensive to implement. It should also be able to handle the inevitable slight variations that are usual in magnetic recording media.
The need for accurate head tracking has long been appreciated and various approaches to this end, including servo controlled head positioning systems, have been proposed with varying degrees of success.
Optical disk drives are capable of achieving track densities in excess of 590 tracks per mm (15,000 tracks per inch). These higher track densities are achieved through the use of closed loop optical servos that allow the read/write head to follow data track eccentricities caused by defects in the medium and by disturbances from outside forces.
Various techniques have been proposed for the modification of magnetic recording materials to provide optical servo information. These techniques can be divided into three broad categories:
The first category encompasses methods of recording optical information that involve physically deforming the magnetic medium, e.g., by stamping, embossing, laser ablation, hole punching, etc. These techniques are disclosed in European Patent Publication Nos. 336491, 351837 and 423662; French Patent Publication No. 2,315,142; U.S. Pat. Nos. 4,371,904, 4,123,788, and 4,315,283; R. E. Acosta et al., in "Floppy Disc Embossing for Servo Applications": IBM Technical Disclosure Bulletin, Vol. 21, No. 10, pp.4259 to 4260 (March 1978); A. S. Hoagland in "Optical Servo of Magnetic Recording": IBM Technical Disclosure Bulletin, Vol. 21, No. 10, pp. 4108 to 4109.(March 1978); International Patent Publication No. WO88/2168; and D.A. Thompson et al., in "Embossed Servo Techniques for Floppy Disks": IERE Conference Proceedings, No. 43, p.321 (July 1978).
The second category encompasses magnetic recording elements in which the optical information takes the form of reflective spots or tracks, either above or below the magnetic recording medium. In most cases, the reflective spots are formed by vacuum deposition of a metal, such as aluminum, e.g., as disclosed in U.S. Pat. Nos. 4,737,877 and 4,558,383, International Patent Publication No. WO88/02168, and French Patent Publication No. 2,315,142.
U.S. Pat. No. 4,633,451 discloses the use of a laser diode to optically servo track a plurality of reflective spots formed on the upper surface of a magnetic recording element. The element comprises a magnetic layer overcoated with a multilayer film, e.g., of amorphous silicon on rhodium.
The third category describes magnetic recording elements in which the servo tracking information is provided by absorptive tracks or spots situated, in most cases, between the magnetic recording medium and the support. Some methods of achieving this are specific to rigid aluminum supports, e.g., as disclosed in International Patent Publication No. WO85/02933 and N. Koshino and S. Ogawa in "Optical Method of Head positioning in Magnetic Disk Systems": IEEE Transactions on Magnetics, pp.631 to 633 (September 1980).
F. P. Laming and H. Mueller in "Optical Storage of Data on a Magnetic Medium": IBM Technical Disclosure Bulletin, Vol. 23, No. 7B, p.3319 (December 1980) disclose the use of optical servo tracks in the form of an absorptive pattern on the upper surface of the magnetic recording layer. The magnetic pigment itself undergoes a color change. It is not clear whether this affects the magnetic properties.
U.S. Pat. No. 4,816,939 discloses magnetic recording materials comprising a transparent support coated on one or both sides with a layer of magnetic recording medium, one side of the support being provided with an optical grating for servo tracking purposes. Different methods of forming the grating are suggested, including photolithographic techniques and photothermal techniques using photothermal materials, silver halide materials, silver transfer materials, photoresist materials etc., as well as techniques such as etching and vapor-deposition. However, in the only embodiment described in any detail, the grating is formed by a silver diffusion process or by photographic exposure and development of a high contrast silver halide film, both of which require wet processing.
Japanese Patent Publication No. 3-201215 discloses magnetic recording elements comprising an optical recording layer interposed between the support and magnetic recording layer. Although reference is made to the optical recording layer containing a dye, this is in the context of ablation-type materials where a laser or similar high intensity exposure source is used to burn away holes or pits in the optical recording layer, the integrity of the layer being destroyed rather than the dye itself.
Although various methods of incorporating optically-readable tracking data have been demonstrated, as exemplified above, the majority of the published literature is concerned primarily with the hardware, such as disk drives, scanning heads etc., rather than the actual magnetic recording materials and the very real problem of producing the relevant materials on a factory scale has not been addressed.
Ideally, any such magnetic recording elements should be compatible with existing disk drives and should be capable of being produced with the minimum of alteration to existing procedures for coating magnetic recording media. Floppy disks are normally cut from a continuous web of coated material and if this already contains the tracking images for the individual disks, the cutting must be performed with a very high degree of precision. Accordingly, the ability to image the magnetic recording elements after (or simultaneously with) the cutting process would be an advantage.
In the light of these requirements, the first category of magnetic recording materials are unsatisfactory, because the stamping or other deformation of the magnetic recording medium may render it unusable with conventional drives, and also sacrifices one of the main advantages of optical tracking, namely the fact that all the magnetic media is potentially available for data storage.
Although the second category of magnetic recording materials do not suffer from these problems, they require vapor-deposition techniques which are costly and difficult to integrate with solvent coating of the magnetic recording media.
The third category of magnetic recording materials is either vague about the method of producing the tracks, e.g., U.S. Pat. No. 4,843,494 discloses magnetic recording elements in which, the optical pattern is "photographically or otherwise formed", but gives no further details, or discloses wet-processed photographic methods that require the imaging to be carried out before coating the magnetic media.
The present invention seeks to provide an alternative method of recording information, such as optical servo tracking information, on magnetic recording materials.